Ground electromagnetic instruments are a sort of instruments that calculate formation resistivity by measuring surface electrical field and magnetic field signals, and are widely applied in mineral exploration, oil and gas survey, subterranean water, and geothermal heat domains, etc. During field operation with a ground electromagnetic method, usually several survey lines are arranged at an equal interval (the distance between two adjacent survey lines is usually hundreds of meters) and several survey points are arranged on each survey line at an equal interval (the spacing between adjacent survey points is usually hundreds of meters), and the instrument carries out measurement at the survey points sequentially, as shown in FIG. 1.
The types, quantities, and positions of the sensors connected with the instrument may vary, depending on the specific measurement method. For example, in the case that a magnetotelluric/audiomagnetotelluric (MT/AMT) method is used, the instrument usually is connected with 4 electric field sensors distributed in 4 orientations of the instrument at equal distance to the instrument, and is connected with 3 magnetic field sensors disposed in a way that the 3 magnetic field sensors are orthogonal to each other, as shown in FIG. 2.
It is seen from the above description: during field operation with a ground electromagnetic instrument, the survey point positioning and orientation problem involved in the survey point arrangement procedure mainly involves determination of positions of survey points on survey lines, i.e., determination of deployment positions of the instrument, and determination of the positions of electric field sensors and the orientations of magnetic field sensors among the sensors connected with the instrument at the survey points.
At present, during field operation, the engineering staff usually manually determine the positions of the survey points of the instrument and the positions and orientations of the sensors connected with the instrument, specifically as follows: before the field operation, the positions and directions of survey lines are planned and designed and the positions of survey points on the survey lines are determined in an indoor environment, and the coordinate values are saved; before the instrument is deployed, persons specially assigned for the work hold a satellite navigation receiver and approach to the preset survey points while observing the coordinate values of the positions; after the instrument is deployed, the engineering staff utilize tools such as handheld compass or forestry compass to determine the directions and carry out setting-out with a tape to determine the deployment points of the electric field sensors, and then use a compass to determine the orientations of the magnetic field sensors, and use a level gauge to determine the levelness.
Generally speaking, the positions of the survey points are found out and the positions and orientations of the sensors are determined manually by the engineering staff. Consequently, the operating efficiency is low, and it is difficult to ensure the accuracy owing to human factors such as the skill and experience of the engineering staff, etc.